This invention relates to a dispenser for dispensing liquids in controlled quantities from a bottle comprising a piston, which is adjustably guided in a cylinder and serves to suck and discharge an adjustable quantity of liquid, wherein said cylinder is secured to a valve block, which contains a suction valve and a discharge valve, a cylindrical sleeve surrounds said cylinder and defines an annular clearance therewith, and said annular clearance contains a stop, which is adjustable to change the length of the stroke of the piston and the quantity of liquid which is to be sucked and to be dispensed, which quantity depends on said stroke, and a fixed stop, which cooperates with said adjustable stop.
A bottle dispenser is known from U.S. Pat. No. 3,729,022. An annular stop is provided, which is an interference fit on the glass cylinder. When the dispenser has been taken apart and the glass cylinder is immersed into hot water, the annular stop can be displaced. The lower portion of the cylindrical sleeve comprises a flexible finger, which is defined by a pair of slots and at its lower end has an inwardly extending flange portion, which constitutes the fixed stop. Because in that known dispenser the piston and the cylindrical sleeve must be removed whenever it is desired to alter the length of the stroke of the piston, and the quantity of liquid which is to be sucked and discharged, which quantity depends on said stroke, there is a desire to provide a dispenser in which the adjustment of the adjustable stop to a desired quantity of liquid can be effected from the outside, as is known from bottle dispensers of a different type. In two known bottle dispensers of the kind in question here, the vertically adjustable stop can be vertically adjusted and fixed in position from the outside. German Patent Specification No. 23 43 687 discloses a bottle dispenser comprising a fixed stop, which remains on a given level and is disposed at that end of the cylinder which is remote from the valve block and the cylindrical sleeve has a longitudinal slot, which has a length that is approximately as large as the distance by which the stroke length of the piston can be adjusted. The adjustable stop is disposed in said slot and because it is segment-shaped it can be adjusted and fixed in position from the outside by means of a set screw, which is provided on the cylindrical sleeve. In the known bottle dispenser, the accuracy (variation from the average value, if the average value is correct) and the correctness (deviation of the average value from the nominal value read from the scale) will always depend on the adjustment of the fixed stop which is disposed at the end of the cylinder and consists of a ring which has been shrunk on the cylinder. When the glass cylinder has been axially shifted in the valve block, the position of the fixed stop relative to the printed scale or the molded rack will no longer be correct. This will also be the case when the fixed stop has been shifted. Besides, the set screw may become loose, particularly after prolonged use, so that the adjustable stop clamped in the longitudinal slot of the cylindrical sleeve comes loose and the adjustable stop which is fixed by the screw is not properly guided. This may result in a change of the correctness and reproducibility of any adjustment during a series of dispensing operations. Another disadvantage resides in that a change of the desired quantity of liquid requires the loosening and subsequent tightening of the set screw so that individual errors may occur in addition to variable system errors. It has now been shown that the known dispenser has substantial sources of error.
In the dispenser known from Laid-open German Application No. 30 31 830, an additional sleeve for guiding the adjustable stop is provided between the cylinder and the cylindrical sleeve and the adjustable stop consists of a helical slider, which extends into an axial groove formed in one of said sleeves and into a helical groove formed in the other of said sleeves. Upon a rotation of the cylindrical sleeve and the guiding sleeve relative to each other, the adjustable stop is raised or lowered like a cam follower, in dependence on the direction of the relative rotation. For this purpose the cylindrical sleeve is held in position with one hand and the other hand is used to rotate the guiding sleeve by means of a handle. That design ensures that the adjustable stop will be reliably held in position in the crossing grooves when the adjustable stop has been adjusted so that the corresponding quantity of liquid will be exactly dispensed even when the bottle dispenser is used very often. The provision of the helical groove permits also an infinite, fine adjustment of different quantities of liquid. The steep helix in which the slider is guided can easily be shifted by a hard shock because the slider is not held in its adjusted position in practice by the component of force which acts in the longitudinal direction of the helix. Small errors of about 0.5% are not detected by the eye. Besides, parts which are movable relative to each other and frictional joints are likely to be subjected to wear, which will increase the backlash.